Roughly about 5'. I was trying to get enough depth and volume to allow critters to live through the winter. The old pond that was there was far too small, and the rubber liner stopped O2 from keeping critters alive. I wound up with dead frogs for the past few years in the spring.
I finished making a large home-brew airstone powered under-gravel filter yesterday, so I lost about 6" of depth due to the gravel needed to cover it over. I think that this will go a long ways to lower muck accumulation by pulling O2 down through the substrate.
I was working on it yesterday, and a medium-sized beautiful corn snake came along and sunned itself on the hot liner and watched me work. I haven't seen one in a few years, but it was like he ways saying, "Hurry up, this is my lunch spot!"
